The present invention relates to a method of controlling transmission apparatus of a vehicle, the apparatus comprising a drive engine, a mechanical transmission connected to the drive engine, and a hydraulic transmission having a pump and n hydraulic motors, where n is greater than or equal to 1, the pump being suitable for being driven by the drive engine for feeding fluid to the hydraulic motors.
It is known, for example from European Patent EP 0 993 982, that combined use can be made of a hydraulic transmission and of a mechanical transmission. The hydraulic transmission is used whenever necessary, in particular for assisting the mechanical transmission under travel conditions that require such assistance, e.g. in the event of loss of grip of at least one of the wheels of the vehicle while it is travelling over slippery terrain, such as the terrain on a building site. Thus, under “normal” conditions of use of the hydraulic assistance, the delivery rate of the pump is determined so as to drive the wheels connected to the hydraulic motors at the same speeds as the wheels connected to the mechanical transmission. Control of the pump thus depends on the information obtained about the speed of the vehicle.
In certain situations, the assistance must be provided at low speed, e.g. in a situation in which the vehicle is starting to move after being at a standstill. Unfortunately, in such situations, the information obtained about the speed of the vehicle can be unreliable because the speed is then too slow to be determined accurately. Therefore, controlling the hydraulic assistance on the basis of the speed of the vehicle cannot be fully effective. The necessary computed data can momentarily be deviant, variable, or erratic. That results in risks of differences between the speeds of the wheels driven by the hydraulic transmission and the speeds of the wheels driven by the mechanical transmission, which differences are detrimental to good drivability of the vehicle. In particular, jolts can take place, with thrust that is too low, or thrust that is too high on the wheels driven by the hydraulic transmission.
An object of the present invention is to solve that difficulty by proposing a method of controlling the hydraulic transmission that provides effective hydraulic assistance, even at low speed.
This object is achieved by the fact that the method comprises performing a “low-speed” assistance stage, during which, while the vehicle is driven by the mechanical transmission, a setpoint pressure for the pressure difference between the feed and the discharge of each of the hydraulic motors is established, the hydraulic motors are fed by the pump, the pressure difference between the feed and the discharge of each of the hydraulic motors is detected, and the delivery rate of the pump is adjusted so that said pressure difference is substantially equal to said setpoint pressure.
Thus, it is on the basis of the setpoint pressure that the delivery rate of the pump is adjusted, without necessarily taking into account the real speed of the vehicle. The wheels coupled to the hydraulic transmission are driven reliably, at a controlled speed that depends on said setpoint pressure. The hydraulic assistance stage is effective and efficient, because the delivery rate of the pump is adjusted to obtain the desired pressure difference, thereby enabling the hydraulic transmission to deliver significant torque, making it possible to control the speeds of the wheels coupled to the hydraulic transmission, and, when necessary, to achieve a controlled increase in speed. The setpoint pressure is determined, as a function of the total torque required, so that it delivers a setpoint hydraulic torque, corresponding to the needs for supplementing the drive torque.
As explained below, it is possible to cause the low-speed assistance stage to cease under various conditions, e.g. when a parameter representative of the speed of the vehicle reaches a determined threshold, and to continue the hydraulic assistance in conventional manner, by taking the speed of the vehicle into account, or else to cause the hydraulic assistance to cease.
Optionally, a starting situation in which the vehicle starts being driven by the mechanical transmission is detected, and the low-speed assistance stage is performed as assistance with the starting.
It is particularly advantageous to use hydraulic assistance as assistance with starting after the vehicle has been at a standstill. In particular when the vehicle is a heavy goods vehicle or a site vehicle, starting it moving requires very high drive torque, which is very demanding on the mechanical transmission and consumes a large amount of energy. In addition, the increase in speed can be slow. The torque available for starting the vehicle is the combination of the drive torque developed by the mechanical transmission and of the hydraulic torque developed by the hydraulic transmission. The hydraulic assistance enables the speed of the vehicle to increase more rapidly and enables starting to be less demanding on the mechanical transmission, resulting in lower energy consumption. The setpoint pressure makes it possible to control the hydraulic transmission reliably, despite the fact that, during starting, the speed of the vehicle is very low.
Optionally, when the starting situation is detected, the assistance with starting remains inactivated so long as the amplitude of actuation of the acceleration control of the vehicle has not reached a target amplitude.
This makes it possible to avoid a situation in which the acceleration control being actuated prior to starting is misinterpreted as a need for assistance with starting. In other words, this avoids a situation in which, while the driver is preparing to start, but before actually starting, the hydraulic transmission delivers unwanted interfering traction. For example, the target amplitude is about 5% of the total stroke of the acceleration control. Optionally, the low-speed assistance stage, and in particular the assistance with starting, is caused to cease when at least one of the following events occurs:                a parameter representative of the speed of the vehicle reaches a predetermined value (this parameter being, for example, the speed of the vehicle or indeed the speeds of the wheels driven by the hydraulic motors, or the speeds of said motors);        the duration of the low-speed assistance stage reaches a predetermined duration;        the outlet torque needed from the mechanical transmission decreases to a predetermined value; and        a clutch connecting the mechanical transmission to the drive engine is released for a predetermined duration, which may be equal to 0, after it has been actuated.        
The low-speed assistance stage is temporary. To cause it to cease, it is possible, for example, to isolate the pump from the feed and discharge ducts of the hydraulic motors, or indeed to place the pump in its zero cylinder capacity configuration, or indeed, as explained below, to make provision for the hydraulic motors to deliver zero torque. The above-mentioned parameters are simple to implement, and are indeed indicative of the vehicle starting to travel under normal conditions, i.e. that the torque required can be delivered by the mechanical transmission on its own. For example, the vehicle speed as from which the low-speed assistance stage is caused to cease may be about 5 kilometers per hour (km/h), or indeed 2 km/h. The predetermined duration may be about 1 minute, or 30 seconds (s), or indeed 10 s. When the parameter used is the needed outlet torque, the predetermined value may be an absolute value, e.g. 4000 newton meters (N·m), or, rather, a percentage of the nominal maximum torque of the mechanical transmission, e.g. 20%.
Optionally, the value of the setpoint pressure is caused to vary during the low-speed assistance stage.
In particular, this variation involves a decrease over time, it being possible for this decrease to be progressive (e.g. linear), optionally after an initial dwell stage during which the setpoint pressure is maintained constant, just at the beginning of the low-speed assistance stage. This variation in the setpoint pressure makes it possible firstly to avoid the speeds at which the wheels are driven being inappropriate for the real instantaneous speed of the vehicle and, in particular, to avoid negative-torque jolts. Secondly, it makes it possible to cause the hydraulic assistance to cease progressively, smoothly and without any sudden drop in the total torque.
Optionally, during the low-speed assistance stage, the value of the setpoint pressure is caused to decrease from a maximum value CPmax to a minimum value CPmin, said minimum value CPmin being reached at the end of the low-speed assistance stage.
Optionally, a setpoint pressure ceiling Pmax is defined and the maximum value for the setpoint pressure CPmax is determined, using the relationship CPmax=f×Pmax, the coefficient f being less than or equal to 1, and being a function of requested characteristics for the low-speed assistance, such as the amplitude of actuation of the acceleration control of the vehicle.
Thus, the variation in the setpoint pressure may be directly related to the acceleration request made by the driver of the vehicle. As the speed of the vehicle approaches the speed desired by the driver for the first gear, the driver tends to release the acceleration control, thereby actually expressing a reduction in the need for torque. The assistance hydraulic torque thus decreases naturally, following the decrease in the need for torque revealed by the reduction in the amplitude of actuation of the acceleration control. It might be said that the aggressiveness of the traction effected by the hydraulic transmission thus depends on the amplitude of the acceleration control, whereas the aggressiveness of the traction effected by the mechanical transmission can depend on the extent to which the clutch control is released.
Optionally, the coefficient f is equal to 1 when the amplitude of actuation of the acceleration control is at least equal to a predetermined percentage of the total stroke of said control, said predetermined percentage being, in particular, in the range 20% to 40%, and more particularly 30%.
As a result, the maximum assistance hydraulic torque is delivered so long as the significant amplitude of actuation of the acceleration control expresses a high need for torque, in particular for accelerating or for starting.
Optionally, the value of the setpoint pressure is maintained at the maximum value CPmax from the beginning of the low-speed assistance stage to the moment at which the drive of the vehicle reaches a determined intermediate state, and then the value of the setpoint pressure is caused to decrease to the minimum value CPmin reached at the end of the low-speed assistance stage.
This intermediate state being reached reveals that the vehicle has started to move in a sufficiently clear and lasting manner for the need for assistance hydraulic torque to start decreasing.
Optionally, the value of the setpoint pressure is caused to decrease using a determined variation relationship, in particular a linear variation relationship.
Optionally, the intermediate state is reached when a parameter representative of the speed of the vehicle reaches a determined intermediate value.
For example, the intermediate target speed is about 2 km/h.
Optionally, activation of the low-speed assistance is enabled or is disabled.
Optionally, during the low-speed assistance stage, the delivery rate of the pump is adjusted by causing the cylinder capacity of said pump to vary.
This is a simple and reliable way of causing the delivery rate of the pump to vary. Another possibility is to cause the speed of the drive engine to vary, but this is difficult at low speed, in particular while the vehicle is starting to move, while said engine is being subjected to high demands for causing the vehicle to start moving. Another possibility, when the pump is connected to the drive engine via an interface making it possible to vary the engine to pump drive ratio, is to cause said ratio to vary.
Optionally, switching-over is performed between a low-speed assistance stage and a hydraulic assistance stage in which the delivery rate of the pump is determined as a function of the outlet speed of the mechanical transmission.
In particular, the n hydraulic motors of the hydraulic transmission are motors having radial pistons. Thus, the or each hydraulic motor may comprise:                a casing with a multi-lobe undulating cam;        a cylinder block, mounted to rotate relative to the cam about an axis of the motor, and having radial cylinders in which the radial pistons slide; and        an internal fluid distributor, prevented from rotating relative to the cam and having distribution ducts suitable for being connected to a fluid feed and to a fluid discharge so that, when the motor is active, it connects the cylinders to the feed and to the discharge in alternation.        
In particular, the distributor has a radial distribution face, in which the distribution ducts open out and which is held in abutment against a radial communication face of the cylinder block, in which face the cylinder ducts open out.
The or each hydraulic motor can be deactivated by declutching its pistons, i.e. by causing the pistons to retract into their cylinders so that they cease to be in contact with the cam. This declutching can be achieved by causing a declutching pressure to prevail in the inside space of the casing, and, optionally by means of return springs associated with the pistons. When the pistons are declutched, the distribution ducts can be put at the same pressure, in particular at a low boost pressure.
In particular, one hydraulic motor is provided for each of the wheels that can be driven by the hydraulic transmission, the rotor of a motor provided for driving a wheel being coupled directly to the wheel so as to drive it at its outlet speed.
In particular, one hydraulic motor is provided for each of the wheels that can be driven by the hydraulic transmission, and said motors are of the rotary cam type, it then being possible for their casings to constitute portions of the hubs of the wheels to which they are coupled.
Thus, in particular as a function of the speed of the vehicle, it is possible to switch from one assistance mode to the other.